An attempt was made to predict western spruce budworm defoliation using egg counts from plots in the Douglas-fir type east of the Continental Divide in Montana, the Douglas-fir type west of the Continental Divide in Montana, and the mixed grand fir Douglas-fir type of northern Idaho. The parameters used in a linear regression model for the three ecotypes were eggs per 100 buds in 1972 as the independent variable and percent defoliation in 1973 as the dependent variable. Correlation coefficients (0 were: 0.39, 0.54, and 0.66 for the data from grand fir in northern Idaho and Douglas-fir in eastern and western Montana respectively. A covariance analysis of the three regression lines showed there were differences between the three ecotypes.